Instead of expending millions of natural human lives, would it not be more beneficial ( economically etc. ) to just create clones with a pre-disposition for their intended job? I don't know how much it costs to create a clone in the 40k universe but I'm sure it's more cost-beneficial then it is today. Also, with the futher advent and use of genetic manipulation, couldn't you make one batch of clones more resistant to certain hostile conditions?
I'm not sure if the subject has been touched upon ( or rigorously chewed over ) so I thought I would give it a shot.